xcode/iOS: support serial number collection via MDM on iOS (#11429)

Fixes tailscale/corp#18366.

This PR provides serial number collection on iOS, by allowing system administrators to pass a `DeviceSerialNumber` MDM key which can be read by the `posture` package in Go.

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

posture/serialnumber_stub.go

@@ -1,14 +1,13 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-// ios: Apple does not allow getting serials on iOS
 // android: not implemented
 // js: not implemented
 // plan9: not implemented
 // solaris: currently unsupported by go-smbios:
 // https://github.com/digitalocean/go-smbios/pull/21
 
-//go:build ios || android || solaris || plan9 || js || wasm || (darwin && !cgo) || tamago || aix
+//go:build android || solaris || plan9 || js || wasm || tamago || aix || (darwin && !cgo && !ios)
 
 package posture